1. Field of the Invention
The present invention pertains to restoring control of a well by regulating pressure in the wellbore while circulating out and killing a kick during drilling operations conducted from a floating vessel or drilling platform at the surface of a body of water.
2. Description of the Prior Art
Drilling operations conducted from floating vessels normally involve the use of marine risers connecting the floating vessel with the wellhead and other equipment on the ocean floor. Such equipment usually includes a blowout preventer control system. The purpose of the blowout preventer system in floating drilling operations is to provide control when a kick occurs and to provide a means of circulating, conditioning, and returning the wellbore to a static condition. Usually, the blowout prevention system includes blowout preventers, a means of controlling release of fluid from the well, and a means of pumping fluid into the well.
Normally, hydrostatic pressure of the drilling fluid column in the well is greater than the pressure of the formation fluid, thus preventing the flow of formation fluids into the wellbore. When a formation with a pressure greater than the hydrostatic pressure in the well is encountered, formation fluids are able to enter the well. The initial influx of formation fluids is commonly referred to as a "kick". As long as hydrostatic pressures control the well, the blowout preventers are in the open position. Should a kick occur, however, blowout prevention equipment and accessories are actuated to close the well.
In most instances, when the blowout preventers are in the closed position due to an occurrence of a kick, additional action is required to restore control of the well. One problem associated with maintaining control can be attributed to the fact that formation fluids entering the well will nearly always contain some gas. This gas is potentially dangerous even if mixed with mud because it can expand greatly when it rises in the well. If the well is shut-in after the entry of appreciable gas and no attempt is made to remove the gas by circulation under controlled conditions, the gas will nevertheless rise in the shut-in well under the influence of gravity, without appreciable expansion. Under these conditions, when the gas reaches the point of shut-in, the pressure at that point may reach such proportions that pressure in the wellbore may result in surface equipment failure, casing failure, or formation breakdown. Such a situation may result in a blowout.
The primary objective in controlling wells which have been invaded by formation fluid is to circulate out any formation fluid influx while maintaining a constant bottomhole pressure slightly greater than the pressure of the formation from the time the well kicks until the weight of the mud in the hole is sufficient to overbalance the formation pressure. To accomplish this objective flow from the annulus is controlled with an adjustable choke so that the pressure of fluid pumped through the drill pipe to circulate out a kick can be controlled to maintain constant bottom hole pressure. By controlling release of fluids from the well the fluid pressure in the well can be regulated to allow for the difference in weight between any heavy fluid being injected into the well and the light mud or gas return fluids and also allow for gas expansion. Controlled release of fluids from the well can also prevent excessive buildup of pressure that may fracture the formation or damage the casing. After the well fluids have passed through the choke, provisions are made for directing the fluids to waste pits, separators, mud tanks, or flares as desired.
In the past, in floating drilling operations controlled release of fluids from the well was accomplished through control lines extending from the blowout preventer or wellhead assembly to the drilling vessel on the surface of the water. Normally control lines would be attached to the outside of the riser assembly. On the floating vessel the high pressure fluids were passed through a choke to regulate the passage of fluid through control lines from below the closed preventer.
In deep water there are several disadvantages in having the wellbore fluid conducted to the vessel in this manner. One disadvantage is the danger that leaks may develop in the control line or in the high pressure choke manifold on the vessel. An uncontrolled release of high pressure fluids on the drilling vessel may endanger the drilling vessel as well as personnel on the vessel. Adverse conditions in the sea will increase the danger that the long, flexible control line may burst or leak. An additional disadvantage in having the high pressure control line extend to the drilling vessel is the problem of imposing an additional pressure on the casing and exposed formation due to dynamic pressure loss in the long control line. A further disadvantage in having high pressure wellbore fluids conducted to the surface may occur with the control line becoming plugged due to the formation of hydrates in the well fluid at the sea floor or before the fluid reaches the surface choke manifold.